Multiple sclerosis is a chronic demyelinating disease affecting primarily people during their active years of life. Long-lasting demyelination leads to axonal degeneration with severe neurological deficits, and in most cases remyelination is limited. Remyelination failure could occur due to the inability of existing oligodendrocytes (OLs) to myelinate demyelinated axons or failure of oligodendrocyte precursor cells (OPCs) to generate myelinating OLs in the lesion. The dynamics of OLs lineage cells is intricately modulated by the local neural activity. OPCs receive synaptic and non-synaptic signals from neurons and undergo depolarization or increase intracellular Ca2+. However, exactly how OPCs sense the level of neuronal activity and initiate a signaling cascade that triggers a terminal differentiation and survival response has remained unclear. In neurons, depolarization-induced Ca2+ entry into axons triggers release of neurotransmitters from synaptic vesicles clustered at the presynaptic terminal by a series of molecular events that involve SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins. OL lineage cells also express various SNARE proteins and transcripts. Our preliminary results of inactivating vesicular SNARE proteins in OPCs and their progeny revealed that SNARE- dependent mechanisms are critical for the proper generation of viable OLs. These observations led us to hypothesize that vesicular SNARE-dependent exocytosis in late OPCs is triggered by neuronal signals and has a critical autocrine function to promote OL differentiation and survival of new OLs. This will be tested by 1) fate analysis of divided OPCs that have defective vesicular SNAREs to determine whether loss of Vesicle-associated membrane protein 2 and/or 3 (VAMP2/3) function compromises OL differentiation and survival (Aim 1); 2) imaging SNARE- containing vesicles and exocytosis events in cultured OPCs and in vivo to determine whether neuronally derived signals promote SNARE-mediated exocytosis and clustering of vesicles (Aim 2); and 3) a proteomics approach to identify the autocrine signal(s) that is released from OPCs in a SNARE-dependent manner (Aim 3). The project will establish experimental evidence for a novel principle regarding the cellular mechanism by which late OPCs trigger their terminal differentiation and survival programs in response to neuronal signals.